1. Field of the Invention
The present invention relates to a high-frequency circuit including an amplifying element. In particular, the present invention relates to a high-frequency circuit including amplifying blocks arranged in multi-stages.
2. Description of the Related Art
Hereinafter, a conventional high-frequency circuit will be described with reference to FIG. 7. FIG. 7 is a circuit diagram showing one example of a conventional high-frequency circuit. FIG. 7 gives an example of the high-frequency circuit in which amplifying blocks are arranged in two stages.
As shown in FIG. 7, a first-stage amplifying block includes an amplifying element 101, a choke inductor 111, and a by-pass capacitor 121. Similarly, a second-stage amplifying block includes an amplifying element 102, a choke inductor 112, and a by-pass capacitor 122.
In the first-stage amplifying block, a power terminal of the amplifying element 101 is connected to one end of the choke inductor 111, the other end of the choke inductor 111 is connected to one end of the by-pass capacitor 121, and the other end of the by-pass capacitor 121 is grounded. Similarly, in the second-stage amplifying block, a power terminal of the amplifying element 102 is connected to one end of the choke inductor 112, the other end of the choke inductor 112 is connected to one end of the by-pass capacitor 122, and the other end of the by-pass capacitor 122 is grounded.
The choke inductors 111 and 112 achieve low impedance with respect to a direct-current component from a common power terminal 143, thereby forming power supply lines to the amplifying elements 101 and 102, respectively. On the other hand, with respect to an alternating-current component from an input terminal 141, the choke inductors 111 and 112 achieve high impedance (an open-circuit state). The by-pass capacitors 121 and 122 ground a high-frequency component (an alternating-current component) from the power supply voltage side of the choke inductors 111 and 112, respectively.
In this high-frequency circuit, a signal input from the input terminal 141 is input to the amplifying element 101 via a matching circuit 131, and is output from the amplifying element 101 after being amplified. The signal output from the amplifying element 101 is then input to the amplifying element 102 via a matching circuit 132, and is output from the amplifying element 102 after being further amplified. The signal output from the amplifying element 102 is then output to the outside of the circuit via a matching circuit 133 and an output terminal 142.
By the way, for efficient signal transmission in a high-frequency circuit, impedance matching is important. The impedance matching is to bring about a condition in which the impedances of two amplifying blocks connected with each other are equal in magnitude but opposite in phase in Smith chart (such a condition is referred to as a xe2x80x9cconjugate matchxe2x80x9d). For example, when an amplifying block A and an amplifying block B are connected with each other, the impedance matching is to adjust the impedance of the amplifying block A to [R+jX (xcexa9)] and the impedance of the amplifying block B to [Rxe2x88x92jX (xcexa9)]. In this case, reflection of signals can be avoided, thus allowing 100% of signals to be transmitted.
In general, in the high-frequency circuit as shown in FIG. 7, the impedance of the signal line is designed to be 50 xcexa9, and the impedance of the input and output also is designed to be 50 xcexa9. However, in many cases, the impedance of the amplifying elements is different from 50 xcexa9. On this account, the matching circuits 131, 132, and 133 are arranged to achieve impedance matching by creating a conjugate match between the impedances of the amplifying blocks, thus allowing efficient signal transmission.
In the high-frequency circuit with the two-stage amplifying blocks shown in FIG. 7, to decrease the number of power terminals, electric power is supplied from the common power terminal 143 to the amplifying elements in the respective amplifying blocks via the choke inductor 111 or 112. This brings about a condition in which the first-stage amplifying block and the second-stage amplifying block are coupled with each other directly. In this case, unless both the by-pass capacitors 121 and 122 have an infinite capacitance, an undesired peak is generated so that a desired frequency characteristic cannot be obtained.
FIG. 8 is a graph showing the result of a simulation performed to examine a frequency characteristic of the conventional high-frequency circuit shown in FIG. 7. In FIG. 8, the horizontal axis indicates a frequency (GHz) from 0.1 (GHz) to 10.1 (GHz) graduated in 1 (GHz) increments. On the other hand, the vertical axis indicates a forward gain [Gain] (dB) from xe2x88x9250 (dB) to 50 (dB) graduated in 10 (dB) increments. In FIG. 8, the mark xe2x80x9cM1xe2x80x9d indicates a forward gain at a design frequency of 5.84 (GHz), and I1 indicates a frequency (GHz) at a point where the measurement is carried out.
As can be seen from FIG. 8, in the high-frequency circuit shown in FIG. 7, a forward gain of about 18.6 (dB) is obtained at the design frequency of 5.84 (GHz). However, as indicated by the mark xe2x80x9cM2xe2x80x9d, after a forward gain of about xe2x88x921.0 (dB) is obtained at a frequency of 2.28 (GHz), the forward gain drops, thereby generating an undesired peak.
The cause of the undesired peak is considered to be as follows. As described above, because the by-pass capacitors 121 and 122 in the respective amplifying blocks have a limited capacitance, sufficient high-frequency grounding is not attained at the frequency at which the undesired peak is generated. More specifically, because the grounding of the high-frequency component from the choke inductors 111 and 112 is not perfect, feedback of high frequency signals is caused between the first-stage amplifying block and the second-stage amplifying block, thereby causing the undesired peak to be generated.
However, the capacitance of the by-pass capacitors 121 and 122 is limited to a certain value as long as the area of the chip is limited. Thus, obtaining an infinite capacitance is almost impossible. On this account, in a high-frequency circuit shown in FIG. 9, a high-frequency separation element 151 is arranged between a first-stage amplifying block and a second-stage amplifying block to prevent the feedback of the high frequency signals from being caused between the first-stage amplifying block and the second-stage amplifying block.
FIG. 9 is a circuit diagram showing another example of a conventional high-frequency circuit. In the high-frequency circuit shown in FIG. 9, the high-frequency separation element 151 is arranged between the first-stage amplifying block and the second-stage amplifying block to prevent an undesired peak. As the high-frequency separation element 151, a xcex/4 line or an inductor generally is used.
In this high-frequency circuit, electric power is supplied to a power terminal of an amplifying element 101 via a common power terminal 143, the high-frequency separation element 151, and a choke inductor 111. On the other hand, electric power is supplied to a power terminal of an amplifying element 102 via a choke inductor 112.
However, the xcex/4 line and the inductor are elements taking up a large area on a chip. Therefore, such elements inhibit the reduction in area of the chip, thereby causing a problem in that the reduction in size of a chip provided with a high-frequency circuit cannot be achieved.
It is an object of the present invention to solve the above-described problem and to provide a high-frequency circuit that can prevent the generation of an undesired peak and contribute to the reduction in area of a chip.
To achieve the above-described object, a high-frequency circuit according to the present invention includes a plurality of amplifying blocks arranged in multi-stages, including at least a first-stage amplifying block and a final-stage amplifying block. Each of the plurality of amplifying blocks includes at least an amplifying element, a choke inductor, and a by-pass capacitor, in which a power terminal of the amplifying element is connected to one end of the choke inductor, the other end of the choke inductor is connected to one end of the by-pass capacitor, and the other end of the by-pass capacitor is grounded. Electric power is supplied from a common power terminal to the amplifying elements in the respective amplifying blocks via the choke inductors in the respective amplifying blocks. A resistive element is provided between the common power terminal and the choke inductor in the amplifying block other than the final-stage amplifying block so that the electric power is supplied to the amplifying element in the amplifying block other than the final-stage amplifying block via the resistive element and the choke inductor, and at least the amplifying elements, the choke inductors, the by-pass capacitors, and the resistive element are provided on a same substrate.
In the high-frequency circuit according to the present invention, a field-effect transistor or a bipolar transistor can be used as the amplifying element. Further, in the high-frequency circuit according to the present invention, it is preferable that a voltage applied to the common power terminal and a resistance of the resistive element are set so that a voltage applied to the amplifying element in the first-stage amplifying block is greater than a knee voltage of the amplifying element in the first-stage amplifying block.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.